w126 upper cam locating bolt is frozen
 

My 1991 560SEL went in for a timing chain and guide change (141K miles but no noise symptoms). My mechanic says that the upper camshaft locating bolt is frozen, and this is the place that the chain guide tool attaches. His strenuous efforts to remove the bolt have come to naught. He tells me that the next choice is to pull the head, rethread the bolt, etc. He did basically removed the front of the engine and says all looks really clean, chain looks good, tensioner well within specs, so he does not believe the situation to be urgent, but this will need to be dealt with at some point. Suggestions?